The present invention relates to a polyester shrinkable film for packaging which, in process of production, shows a good handling property in drying and, as a finished product, is excellent in both the heat-sealing property and the shrinking property.
At present, shrinkable film of polyvinyl chloride and paper are widely used as materials for the labels on containers. As containers for refreshing beverages, however, bottles made of polyethylene terephthalate (PET) have been finding growing acceptance in addition to glass bottles. To encourage recovery of used PET bottles for re-use, the desirability of using labels made of polyester shrinkable film on the PET bottles has been finding wide recognition.
In answer to this desire, it has been proposed to obtain a film suitable for the aforementioned labels by extruding an amorphous copolyester or a composition of the amorphous copolyester and PET or polybutylene terephthalate (PBT) into the form of a sheet and monoaxially stretching the extruded sheet in the longitudinal direction or the transverse direction (as disclosed in the specifications of Japanese Patent Application Laying-Open (KOKAI) No. 57-42,726 (1982), Japanese Patent Application Laying-Open (KOKAI) No. 57-159,618 (1982), and Japanese Patent Application Laying-Open (KOKAI) No. 59-97,175 (1984).
The copolyester hitherto proposed for the production of the film under discussion, however, is amorphous and has a second-order transition temperature of about 80.degree. C. When such polyester is dried at a temperature exceeding 80.degree. C., the polyester is fused to be a form of a conglomerate and must be dried overnight under a vacuum at a low temperature of about 60.degree. C. When the production of the film is to be carried out on a commercial scale, the drying effected in the manner just described is not practicable because the drying consumes an unduly long time and, as the inevitably result, the continuous conversion of the polyester into the film necessitates the use of an unusually large continuous or batchwise vacuum drying device.
As the results of research for solving the problem in drying, the present inventors have consequently found that a composition of a specific crystalline polyester and PET, PBT, etc. can be dried under ordinary conditions without any difficulty and any particular device therefor and, in addition to the above, a film produced from such a composition is excellent in both the heat-sealing property and the shrinking property. On the basis of this findings, the present inventors have accomplished the present invention.